[tz] Extended data format

[Andy Lipscomb]

"I guess such =zone references should refer to a zone earlier in the same file to keep things simple, i.e. no forward or external references."

Not an absolute logical necessity, but certainly a safe way to do things. It does have implications in the design of the extended files, in that they would need to be supersets of the basic files, not supplemental files used in conjunction with them. In the alternative, a limited form of import could be allowed in that a file named (for exampile) "kalimdor-extended" would be allowed to reference the zones of the file "kalimdor" as well as the ones it contains itself.